Why buy the cow when you can get the milk for free? Will you buy the cow if there's a milk substitute on the market? Is buying even a good idea or will a lease do? And what does the cow think about all of this?

Happily for those who prefer not to equate women with barnyard animals, that particular metaphor has largely fallen out of use. But the questions underlying it endure. Mark Regnerus, a sociologist at the University of Texas at Austin, sets out to address them in his alarmingly titled new book, "Cheap Sex: The Transformation of Men, Marriage, and Monogamy."

Regnerus's answers are not particularly uplifting. But neither will they be surprising to anyone who has observed the world of mating and dating over the past decade or so. Still, the frank way that Regnerus delivers his findings feels unusually bracing. And while there's room to question his conclusions, Regnerus asks us to first set aside our idealistic blinders and hear him out.

So let's try.

The "cheap sex" of the book's title refers to the way that sexual acts have become radically less costly over the past half century, in economic and social terms. Widespread contraception makes sex less risky, online dating makes it more accessible and high-quality pornography is easy to find if you can't avail yourself of the real thing.

But such radical change is rarely without consequence. To measure it in the realm of our relationships, Regnerus relies on the concept of sexual economics, in which mating is seen as a marketplace. In this view, women are gatekeepers to a limited, highly desired product: sex. In exchange for access to this product, men proffer commitment, fidelity and resources. Regnerus believes that the sharp drop in the value of sex has shifted the market, even its more conservative parts, leading to a massive overall slowdown in the creation of committed relationships like marriage, in large part because men see less of a need to make themselves into appealing long-term partners.

Of course, there are many reasons besides sex to enter into a committed relationship, and most people eventually do — it just takes them longer now. Unfortunately, however, this lengthened timeline doesn't always align with women's preferences. One piece of evidence: Births from assisted reproductive technology, often a last resort as fertility wanes or a suitable partner can't be found, jumped by 70 percent between 2000 and 2013.

Regnerus's findings may not be in line with our egalitarian ambitions, a fact that critics of his book have pointed out, vociferously. But how things should be often has little bearing on how things are.

Yes, advances in education, the workplace and the sexual realm have helped women in incontrovertible ways. Marriage is no longer the sole key to a good life, and we have time and freedom to choose. Yet among younger women, especially those who want that sort of traditional relationship, there increasingly seems to be a vague dissatisfaction with the state of things. Why, when women have gained so much power, are we so often at impasse in our romantic relationships? Why do men our age seem so unmotivated to grow up and so ambivalent about committing? As uncomfortable as it may be to contemplate, the shifts this book describes may provide an inkling of an explanation.

Regnerus states his findings baldly, and he has little patience for those who would prefer to focus on the upsides of such shifts. When it comes to commonly held modern ideals — of gender egalitarianism, individualism, the assumption that men might seek to improve themselves even without outside prodding — his response is skepticism bordering on exasperation. "In the domain of sex and relationships, men will act as nobly as women collectively demand," he writes. "This is an aggravating statement for women to read, no doubt. They do not want to be responsible for 'raising' men. But it is realistic."

A jaundiced eye, to be sure, but before dismissing it outright we might try it out on Hugh Hefner, perhaps America's foremost advocate for cheap sex, and the way his story has been told since his death last week at age 91. He was a champion of sexual liberation, we're told. An activist for freedom of all kinds! These principles feel right to celebrate, and many eulogists have. But fewer seem willing to prod the trade-offs inherent in what Hefner promoted. Has all this sexual liberation made it easier for some to prey on others? And more deeply: How does a focus on individual fulfillment change the way we function as a society?

Throughout his book, Regnerus prods the reader to be skeptical of utopianism and see the world as it is. It's a useful, if unpleasant, reminder for an era in which our goals seem both loftier and further out of reach than ever. Sex may be cheap now, but talk has always been.

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Christine EmbaChristine Emba is an opinion columnist and editor for The Post. Before coming to The Post in 2015, Christine was the Hilton Kramer Fellow in Criticism at the New Criterion and a deputy editor at the Economist Intelligence Unit. Follow